This invention relates to the process of wire bonding in the production of semiconductor devices, wherein extremely thin conductor wires are bonded between contact pads on the semiconductor chip and relatively large external connector leads on a lead frame.
The modern assembly process for microcomponent semiconductor devices is largely automated to increase productivity by reducing cost and increasing assembly rates. In this automated process, the external leads for connecting the semiconductor device into a circuit are contained on a flexible strip of lead frames. Each frame of the strip contains the external leads for one semiconductor device. The frame also contains a supporting platform, or "paddle", on which the semiconductor chip will be attached. Thin conducting wires are attached between conductor pads on the chip and the external leads of the frame. The support portions of the frame are then cut away, leaving the external leads and the mounted semiconductor as a finished circuit element.
The automated process in such assembly is usually carried out on a single assembly machine, and involves sequencing the lead frame strip through the machine in steps of one frame-width each. At selected positions in the sequence machine tools affix the semiconductor chip to the support paddle, bond the thin wires between the chip and the external leads, cut away the support structure of the frame, and bend the external leads into a desired configuration.
This invention relates particularly to the step in the above process wherein the thin conductor wires are bonded between the semiconductor chip and the external leads. These thin wires are often flexible gold alloy having extremely thin diameters relative to their length. They are normally applied through a moving capillary tube from the mouth of which the connecting wire emerges. The capillary tube is mounted on an automated arm which controls the path of the tube and thus the placement of the wires. The path is normally such that the capillary tube reciprocates downward to place a portion of gold wire on a contact pad of the semiconductor chip where it is bonded thereto by known wire bonding means. The capillary tube then moves upward and across to the external leads, whereby the wire is drawn from the mouth of the tube. The capillary then reciprocates downward again to place the wire on the external leads, where it is severed and bonded to the lead.
One of the problems inherent in this capillary bonding process arises from the flexibility of the thin gold wire. As gold is relatively flexible, and the wire has an extremely small diameter relative to its length, the wire may easily sag and become short circuited on adjacent wires or the semiconductor chip, or even become fouled with projecting edges of the assembly machine as the film strip is advanced.
Possible solutions to the above problem include increasing the diameter of the gold wire, or of shortening the length of the gold wires by reducing the distance between the semiconductor chip and the tips of the external leads. Both of these solutions, however, have economic liabilities. Increasing the diameter of the wire means that an increased amount of gold must be used. Changing the dimensions of the external lead frame means that a standardized lead frame cannot be used. Instead, a new lead frame must be used for each size semiconductor chip. This would entail extensive retooling costs that are preferably avoided. Moreover, where there are a large number of external leads on the frame, there is a limit to how much the leads can be elongated toward the frame center without reducing the spacing between leads excessively.
The present invention relates to a straightforward yet effective means of preventing this undesirable sag of the thin gold wires, yet allows the use of a relatively small diameter wire and of standardized lead frames. The details of this invention and the manner in which it accomplishes the above objectives will become apparent upon reading the descriptions which follow.